[2.01] Brown Dwarf Spectra from 3-4 Microns: Searches for Methane and Other Molecules

Using the CGS4 Spectrometer on the United Kingdom Infrared
Telescope (UKIRT) on Mauna Kea, we have measured the spectra
of four brown dwarfs, Kelu 1 (Ruiz et al. 1997), and
DENIS-P~J1228.2-1547, DENIS-P~J1058.7-1548, and
DENIS-P~J0205.4-1159 (Delfosse et al. 1997) in the
previously unobserved interval between 3 and 4 microns. We
have also measured the spectra of several late M dwarfs. Our
primary motive for these observations was to search for the
strong methane fundamental at 3.3 \mum. Because methane is
predicted to become thermochemically favored only at
temperatures below about 1600~K (Fegley and Lodders, 1997)
and because any object this cool is almost certainly
substellar, the presence of methane is a powerful proxy
indicator of brown dwarf status. None of the objects
measured show obvious signs of CH4 absorption similar to
that seen in Gl~229B and the recently detected field
T-dwarfs suggesting that they have effective temperatures
somewhat higher than currently estimated. We have also
searched for HDO, H2S and HCl in the spectra and will
report on the results of this search.